Coating by the use of the slide hopper, which is usually useful for forming a thin layer, is widely used to manufacture photographic materials. Since the photographic material generally has a structure composed of 10 or more layers within a total thickness of tens .mu.m, the slightest change in coating thickness of each layer has a large effect on quality of the photographic material so that control of the coating thickness is very important.
In recent years, formation of a thinner layer has been increasingly expected. In order to attain stable, high-speed coating with an organic solvent by the use of the slide hopper to form a thin layer, the gap between a lip of the hopper and the web has to be narrowed and maintained with high precision.
For example, when a coating solution containing acetone and methanol as main components is applied by the use of a slide hopper with a coating width of 1 m, even if the lip has been set in parallel with the backing roller prior to feed of the coating solution, evaporation of the coating solution reduces temperatures of the members such as the slide surface on feeding the coating solution, causing the whole hopper to be concaved in relation to the backing roller. As a result, the gap between the lip and the backing roller may change by as much as about 40 .mu.m. Since the gap is usually set within the range of 50 to 300 .mu.m, the changes up to 40 .mu.m have an important effect on coating properties under thin-layer coating conditions where narrowed and precise setting of the gap is particularly required.
Thus, in the coating by the use of the slide hopper, a major cause of fluctuations in-coating thickness consists in fluctuations in the gap between the lip and a substrate to be coated. The reason for the fluctuations in the gap between the lip and the substrate is that the coating with an organic solvent-based coating solution is accompanied by the evaporation of the organic solvent when the coating solution flows through the slide surface of the slide hopper, and therefore, that the heat of evaporation partially reduces the temperature of the lip, causing contraction thereof.
As one of the conventional techniques to prevent such heat contraction of the slide hopper, use of coating apparatus made of ceramics is described in U.S. Pat. No. 5,275,660 (corresponding to JP-A-2-71869, The term "JP-A" as used herein means an "unexamined published Japanese patent application"). This technique helps to reduce the heat contraction of the hopper because of smaller coefficients of linear expansion of the ceramics, compared to stainless steel which is widely used as a raw material for the hopper.
It is further described in U.S. Pat. No. 4,292,349 (corresponding to JP-A-55-75758) that the coating section is provided with a cover to form an airtight region therein.
Furthermore, JP-A-62-53768 proposes to extremely reduce a length of the slide surface (a distance from an outlet for the coating solution to the lip) to from 0.1 to 10 mm to prevent the evaporation of the organic solvent flowing through the slide surface. This helps to prevent the fluctuations in coating thickness stemming from a flow of the coating solution depending upon non-uniform surface tension developed between the slit and the lip by the evaporation of the organic solvent flowing through the slide surface and from changes in physical properties of the flowing coating solution such as viscosity in the thickness direction.
However, these conventional techniques described above have the following disadvantages, respectively.
The technique described in U.S. Pat. No. 5,275,660 has difficulty in cutting the ceramics because of extremely large hardness thereof. The ceramics are only cut by a few .mu.m in a single operation so that the cutting work requires much time. Some problems such as breakage in cutting tools and cracking in the raw materials may further be encountered.
In the technique described in U.S. Pat. No. 4,292,349, it is impossible to keep the coating section completely airtight. Particularly, movement of a substrate in the vicinity of the bead disturbs the airtightness within the hopper. This makes it impossible to completely prevent the evaporation of the organic solvent flowing through the slide surface so that contraction causes the lip to be deformed, failing to form a thin layer with high precision.
The technique described in JP-A-62-53768 also helps to prevent the evaporation of the organic solvent flowing through the slide surface. However, a shortened length of the slide surface causes insufficient flowing speed of the coating solution so that some types of coating solutions may not be applied. This technique therefore lacks flexibility in use. Further, a reduction in cross-sectional area of the hopper due to the shortened slide surface lowers stiffness to the deflection stemming from changes in temperature, which makes it difficult to maintain the gap between the lip and the web with high precision.
Corrosion of the lip also causes the fluctuations in coating thickness. The reasons for the corrosion are that the coating solution may possibly contain a liquid extremely low in pH or a corrosive substance and that a strongly corrosive liquid may be inevitably used for washing the coating apparatus.
Further, heat generated by working each block of the hopper causes distortion. Use of raw materials with relatively large coefficients of linear expansion may possibly result in reduction in working precision to an extent that the precision itself cannot be neglected.